bottom-border-1px($color)
    position relative
    border none
    &:after
        content ''
        position absolute
        left 0
        bottom 0
        width 100% 
        height 1px
        background-color $color
        transform scaleY(0.5)
top-border-1px($color)
        position relative
        &::before
            content ''
            position absolute
            z-index 200
            left 0
            top 0
            width 100%
            height 1px
            background-color $color			
//根据像素比缩放1px 像素边框
@media only screen and (-webkit-device-pixel-ratio: 2)
	.border-1px
		&::before
			transform scaleY(.5)
@media only screen and (-webkit-device-pixel-ratio: 3)
	.border-1px
		&::before
			transform scaleY(.333333)
//根据像素比来使用2x图 3x图
bg-image($url)
    background-image: url($url + "@2x.png")
    @media (-webkit-min-device-ratio: 3),(min-device-pixel-ratio: 3)
        background-image: url($url + "@3x.png")
// 清除浮动
clearFix()
    *zoom 1
    &::after
        content ''
        display block
        clear both